The present invention is generally related to the analysis of electromagnetic waves and more particularly, is related to a system and method for analyzing the intensity and phase of light pulses with durations shorter than one nanosecond.
Analysis of electromagnetic waves, such as ultrashort laser pulses, is required for fundamental research and applications, such as laser development, semiconductor characterization, combustion diagnostics, and optical coherence tomography. In addition, many material characterization techniques depend upon precise analysis of ultrashort pulses. One of the most promising applications requiring analysis of such waves is communications using intensity-shaped pulses and/or phase-shaped pulses.
For decades autocorrelators were the primary tool used to measure electromagnetic waves such as ultrashort laser pulses. However, autocorrelators are complex instruments having a large number of components, and autocorrelators yield, at best, only vague measurements of the pulse. To measure a pulse with an autocorrelator, the pulse is split into two identical copies of the pulse. The two pulses are then spatially and temporally overlapped in a carefully aligned nonlinear optical medium such as a second-harmonic-generation (SHG) crystal. The relative delay between the pulses must be scanned while maintaining alignment. The alignment involves four sensitive degrees of freedomxe2x80x94two spatial, one temporal, and one crystal angle. The sensitivity of the alignment increases the potential for error in autocorrelators and other pulse analysis systems.
In addition, autocorrelators require a very thin SHG crystal due to bandwidth constraints. The required thin SHG crystals can be expensive, hard to align, difficult to obtain, and troublesome to handle. The alignment and handling requirements of the SHG crystal increase the complexity of the autocorrelator. Thus, the thin SHG crystal is also a potential source of error in pulse analysis systems.
The potential source of error related to thin SHG crystals arises from the need to avoid group velocity mismatch (GVM). GVM is the walking off in time of the pulse and the second harmonic of the pulse due to different group velocities of the wavelength of the pulse and the wavelength of the second harmonic of the pulse. GVM can also be described as a crystal having a finite phase-matching bandwidth that only allows a small range of wavelengths to achieve efficient frequency doubling. The thin SHG crystal must be thin enough that the two pulses overlap throughout the entire thin SHG crystal. As an example, analysis of 100 femtosecond pulses requires an SHG crystal with a thickness of approximately 100 microns. SHG crystals approximately 100 microns thick are difficult to obtain and the thickness is difficult to verify. In addition, SHG crystal efficiency scales as the square of the SHG crystal thickness. Therefore, even when the SHG crystal is sufficiently thin, poor signal strength can limit the sensitivity of the analysis.
Autocorrelators do not measure the full intensity and phase of a pulse. One way to measure the full intensity and phase of an ultrashort laser pulse is a method known as Frequency-Resolved Optical Gating (FROG). FROG is described in U.S. Pat. No. 5,754,292 to Kane and Trebino, and U.S. Pat. No. 5,530,544 to Trebino et al. The ""292 and ""544 Patents are entirely incorporated herein by reference. The FROG method adds a spectrometer to an autocorrelator. Unfortunately, the addition of the spectrometer further complicates the autocorrelator and increases alignment problems. However, alternatives to FROG are more complex than the FROG apparatus and method. Some of the alternatives require one or more interferometers, or a first interferometer within a second interferometer.
FIG. 1 is a schematic illustration of a prior art ultrashort full pulse measuring device 10 using the FROG methodology. In FIG. 1 an ultrashort light input pulse 12 is introduced to a beam splitter 14. The beam splitter 14 produces a probe pulse 13 and a gate pulse 15. The probe pulse 13 is directed by an optical alignment system 16 through lens 20 into a rapidly responding nonlinear optical medium such as a thin SHG crystal 22. The gate pulse 15 is provided with a variable delay xe2x80x9cxcfx84xe2x80x9d by delay line 18. The probe pulse 13 and the gate pulse 15 are focused into the thin SHG crystal 22 through lens 20. Thus, beams having electric fields E(t) and E(t-xcfx84) intersect in the thin SHG crystal 22.
The interaction of the two beams in the thin SHG crystal 22 can occur via many processes and geometries, and many are treated in the prior art including Laser-Induced Dynamic Gratings, by H. J. Eichler et al., Springer-Verlag, New York (1988), which is entirely incorporated herein by reference. Several such geometries are shown in the ""544 Patent. In the geometry shown in FIG. 1, the thin SHG crystal 22 is phase-matched for noncollinear second harmonic generation. With the thin SHG crystal 22, neither the probe pulse 13 nor the gate pulse 15 alone achieves significant second harmonic generation in the direction of the signal pulse. However, the probe pulse 13 and the gate pulse 15 together do achieve efficient second order harmonic generation. The gate pulse 15 gates the probe pulse 13 (i.e., gates a temporal slice of the probe pulse 13). The roles of the probe pulse 13 and the gate pulse 15 may be reversed. It does not matter which pulse is considered as gating the other.
Still referring to FIG. 1, a signal pulse (Esig(t,xcfx84)) 17 is directed to a wavelength-selection device, such as a spectrometer 24, to resolve the frequency components in the signal pulse 17. The signal pulse 17 includes selected temporal slices of the probe pulse 13. A camera 26 records the spectrum of the input pulse 12 as a function of the time delay of the probe pulse 13 to produce an intensity plot vs. frequency (or wavelength) and delay, i.e. the xe2x80x9ctracexe2x80x9d of the input pulse 12. The input pulse 12 may be a femtosecond pulse; a negatively chirped pulse (i.e., a pulse with decreasing frequency with time); an unchirped pulse (i.e., a constant frequency pulse); a positively chirped pulse (i.e., a pulse with increasing frequency with time); or any other pulse. The camera 26 records traces corresponding to the input pulse 12 to uniquely determine the intensity and phase characteristics of the input pulse 12. The trace is a plot of intensity vs. frequency and delay (i.e., a type of spectrogram of the pulse) that is familiar to those of ordinary skill in the art. The trace contains all of the information necessary to reconstruct the intensity and phase characteristics of the input pulse 12. Without the spectrometer 24, the ultrashort pulse measuring device 10 of FIG. 1 is an autocorrelator in which the signal field""s energy is measured vs. delay.
The trace is recorded and provided to a processing unit 28 to carry out processing calculations such as those described in the ""544 Patent. Such a processing unit 28 may be a digital computer operating in accordance with a stored program, a neural net which is trained to recognize the output of the spectrometer 24, or numerous other calculating devices known to those skilled in the art, some of which are shown in the ""544 Patent.
The operation and features of the prior art analysis systems have been described in various articles including, xe2x80x9cMeasuring Ultrashort Laser Pulses in the Time-Frequency Domain Using Frequency-Resolved Optical Gating,xe2x80x9d Rev. Sci. Instr., vol. 68, pp. 3277-3295, 1997 by R. Trebino, K. W. DeLong, D. N. Fittinghoff, J. N. Sweetser, M. A. Krumbxc3xcgel, and D. J. Kane, which is entirely incorporated herein by reference. Prior art analysis systems are also described in U.S. Pat. No. 5,648,866, and U.S. Pat. No. 6,008,899, both to Trebino et al, which are entirely incorporated herein by reference. In addition, prior art analysis systems are described in U.S. Pat. No. 5,936,732 to Smirl et al, and U.S Pat. No. 5,754,292 to Kane et al, which are also entirely incorporated herein by reference.
Despite the advances noted in the field, potential sources of error remain related to the sensitivity of the alignment of the prior art analysis devices and the very thin SHG crystals. In addition, due to their complexity, these devices are expensive (approximately $10,000), are fairly large, and are not easy to use. Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
An electromagnetic wave analyzer provides systems and methods for measuring and/or detecting the intensity and phase of light pulses. The electromagnetic wave analyzer eliminates the need for a spectrometer, a beam-splitter, a delay line, and the thin second-harmonic-generation (SHG) crystal. The electromagnetic wave analyzer uses an easily attainable thick SHG crystal and yields about one thousand times more signal than the prior art analysis devices. In addition, the electromagnetic wave analyzer does not require sensitive alignment parameters. The electromagnetic wave analyzer can measure a single pulse of an electromagnetic wave by using a few optical components. Yet, the electromagnetic wave analyzer yields traces as useful or better than those of the prior art.
Briefly described, in architecture, the electromagnetic wave analyzer can be implemented as follows. The electromagnetic wave may be an ultrafast laser pulse. A Fresnel biprism accepts the ultrafast laser input pulse and produces a probe pulse and a gate pulse. The Fresnel biprism then delays the gate pulse in relation to the probe pulse so that the relative delay varies with the transverse position of a nonlinear optical medium, such as a second harmonic generation (SHG) crystal. The SHG crystal is configured to accept the probe pulse and the gate pulse. The SHG crystal has a group velocity mismatch (GVM) and a group velocity dispersion (GVD) in relation to the input pulse. A pulse time-bandwidth product (TBP) may be used to determine the GVM and the GVD. The SHG crystal is configured so that the pulse TBP is less than GVM/GVD. The GVM and GVD may be varied by varying the thickness of the SHG crystal and/or the crystal material. The SHG crystal phase-matches a limited range of frequencies. The phase-matching may be a frequency doubling of the limited range of frequencies. The SHG crystal produces an output based on the phase-matching of the limited range of frequencies. A camera then records a data trace representing intensity vs. delay in a first direction and frequency in a second direction. The second direction is generally near perpendicular to the first direction.
The electromagnetic wave analyzer can also be viewed as providing a method for detecting electromagnetic waves such as ultrashort laser pulses. In this regard, the method can be broadly summarized by the following steps: providing an input pulse; producing a probe pulse and a gated pulse; delaying the gate pulse in relation to the probe pulse; introducing the probe pulse and the gated pulse to a nonlinear optical medium such as an SHG crystal, the SHG crystal having a GVM and a GVD in relation to the input pulse, the input pulse having a TBP where TBP is less than GVM/GVD, the SHG crystal being configured to phase-match a limited range of frequencies and to produce an output based on the phase-matching; and introducing the output of the SHG crystal to a camera that detects a trace that yields the intensity and phase of the input pulse.